The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive having low viscosity and showing good cohesion level like high initial bond resistance that may be applied at relatively low temperatures for example for making elastic components such as laminates containing elastic strands for use in disposable diapers.
The increasing complexity of manufactured goods, in particular disposable goods, also leads to major improvements and developments in the hot melt adhesive industry. Hot melt adhesives are being used to bond a wider variety of substrates, within a broader adhesive application process window, and for a large end-use portfolio. For example considering the diaper manufacturing industry, materials involved may be non-woven materials, polymeric films, and in general elastomeric components. These elastomeric components can be used in products like diapers, in a form of strands, films, nonwovens or any other continuous or discrete form.
Processability of hot melt adhesive is linked to their ability to be melted, and transported and/or coated in a molten stage to the final location where the bond is required. Usually the molten adhesive is sprayed, or coated as a film. Once cooled down, the adhesive needs to fulfill multiple requirements, like bond strength measured by peel force or bond retention under or after mechanical stress, and under or after various thermal conditions.
Typically hot melt adhesives can be based on polymers such as polyolefins (ethylene- or propene-based polymers), or functionalized polyolefins (ethylene or propene copolymers with oxygenated function containing monomers), or styrene block copolymers containing at least one rubbery phase, like SIS, or SBS. Styrene block copolymers are of interest due to their dual characteristics, i.e. cohesion of the styrenic phase associated with the rubber behavior of another phase. Typical application temperatures are equal to or higher than 150° C.
Combining parameters in the areas of a substrate's nature, adhesive processability and a product's end use requirements, there has been a steady trend in the industry to change and use more sophisticated substrate types, for technical or economical reasons. This can lead to the use of more sensitive substrate materials, in terms of mechanical, thermal, weather or time resistance, with the need to not compromise any of the other attributes, i.e. the overall manufacturing process should remain of the same concept, and the end use of the item should be fulfilled in the same way, or enhanced. For example in the diaper industry, typical application temperatures for elastic attachment would be around 163° C. Depending on the bonding performances required, however, it may be higher. Lowering the application temperature presents problems in terms of wet-out, and most of the time 150° C. would be seen as a minimum temperature one can go to attach elastic parts onto the diaper structure.
It is known in the diaper industry that the use of heat sensitive substrates may cause problems if the adhesive temperature is too high because the production line has to be stopped each time the substrate breaks or is damaged by the molten adhesive material (described as a “burn through” phenomenon) and would need to be replaced or fixed before starting the line again. This may also be the case with non-woven substrates or with elastomeric components used in the diaper structure. Thus, a lower application temperature of the hot melt adhesive would be very helpful to avoid maintenance issues and downtime on production lines.
Another factor making it desirable to reduce the application temperatures of hot melt adhesives is that the diaper industry has been trying to use thinner gauge films in order to decrease the overall diaper's material weight, and consequently the material cost. Over the years, this has been achieved with more or less success, depending on the difficulty to keep both the manufacturing process and end-use attributes the same. Heat distortion or deterioration of the film or non-woven substrates can occur easily when the hot adhesive material contacts the substrates' surfaces. As a result, the functionality of the substrates in the end-use structure is affected in a way that is not acceptable. Among other reasons to decrease the application temperature of the hot melt adhesives is the concern of saving some energy cost needed to heat the adhesive material, as well as the need to enhance safety for the workers on the production line to minimize potential burn hazards.
Many references offer possible solutions to apply a hot melt adhesive material at low temperature. Lowering material viscosity is very often seen as the only criterion to lower the application temperature. Both lack of cohesion and incompatibility of composition ingredients, however, have hindered solving this problem in the manner in which the present invention proposes to solve the problem.
It has to be noted that the phrase “low application temperature hot melt adhesive” as used herein corresponds to the ability to apply the molten or deformable adhesive material at a relatively low process temperature, or “application temperature”, i.e. less than 150° C., in order to build a bond between two substrates. Sometimes prior art references utilize the phrase “low temperature” as a term to qualify adhesive materials presenting good mechanical and adhesive performances at low temperatures into the finished good once the bonding process has been made. These low temperatures are usually lower than room temperature, but it is not the intent of the present invention to deal with this specific requirement.
Adhesive application at low temperature is relatively easily achieved for specific applications or application domains where there is no harsh cohesion required. Although focus could be put on Shear Adhesion Failure Temperature (SAFT) value, the aim of this test is more in defining a failure under a constantly increasing temperature than reflecting the mechanical resistance of the bond over time. Many references exhibit interesting SAFT values that do not correlate with the ability of the adhesive materials to resist creep conditions over an extended period of more than few minutes at elevated temperature.
For example, U.S. Pat. No. 6,180,229 B1 is focused on the very narrow process engineering domain of a screen roller or an engraved roller to provide discontinuous coatings of any Hot Melt Pressure Sensitive Adhesive (HMPSA). The application areas are feminine napkin, bandages, tapes, where intended internal cohesion of the adhesive does not need to be high, as the described formulae examples contain more than 30% of an oil. Using this amount of oil at the viscosity level claimed, i.e. less than 5,000 mPa·s at 125° C., it is clear that this reference does not teach how to achieve a conventional elastic attachment at low temperature. No mention is made about spraying the adhesive material at low temperature on elastic strands and in-between thin film substrates in order to hold these elastic strands in place over time.
EP 0 451 919 B1 and EP 0 451 920 also do not mention any potential for the adhesive materials to hold elastic strands in a diaper structure. Although they mention that maintaining adhesion in elastically demanding applications is generally the role of styrenic block copolymer based adhesives, there is no discussion in either of these references about how to achieve a conventional elastic attachment at low application temperature.
U.S. Pat. No. 5,275,589 describes how to bond a polyolefinic film to a non-woven substrate to achieve the construction of what is known in the diaper industry as a cloth-like back sheet. This reference describes a coating process with an adhesive containing substantially no oil. Even if the application temperature was low, the viscosity level of the examples described in this patent would be very high and would thus hinder any process where the adhesive needed to be pumped and pushed through conventional components of a hot melt adhesive application device.
U.S. Pat. No. 6,465,557 B1 claims an adhesive that can be used at low temperature. The potential uses for the adhesive set forth is the description is clearly remote from any higher cohesion demanding application, i.e. the adhesive is stated to be useful for being applied to a release liner and transfer coated to a garment, for use in a feminine pad, panty shield, or diaper inserts. As such, these adhesives are pressure sensitive adhesives (PSA).
EP 0 798 358 B1 is focused on bottle labeling applications, where elevated temperature storage conditions and long open times are required. Again, this does not help to get bond retention in the way needed for elastic attachment in a diaper structure.
U.S. Pat. No. 6,818,093 B1 is very specific to construction applications in which dermatologically-compatible coatings are present on substrates. This reference proposes an interesting way to enhance the adhesion level of adhesives, as it is well know that the dermatological compatible coating affects the surface nature of the substrates and the ageing effect of adhesive bonding. Although this reference mentions that hot melt adhesives for structural or elastic attachment are available on the market, it does not provide any solution to applying them at low temperatures for elastic attachment.
WO 97/10310 mentions adhesive systems that can be applied at temperatures as low as 121° C., and having a very high diblock content in the polymer component. This is typical of non-cohesive systems in that the diblock structure provides a tendency for the adhesive material to creep upon ageing, due to temperature or mechanical stress.
WO 00/78886A1 mentions applications at low temperatures of 130° C. to 135° C. Unfortunately, application results are exhibited only for spiral construction or bottle labeling, which are non-demanding applications in terms of cohesion and bond retention, in contrast to the ones needed in an elastic attachment environment. A cohesive adhesive formula is shown in an example, but its viscosity level at 135° C. does not lead one to think this material can be easily applied at this temperature or below.
Whenever the focus is to obtain cohesion for bond retention or creep resistance, for example for elastic attachment in a diaper structure, the sophistication level into the adhesive formulation needed to achieve this goal is high, and systematically not reached in the way this present invention presents it. For example, U.S. Pat. No. 6,180,229 B1 proposes to coat adhesives at temperatures ranging from 90° C. to 140° C. It describes formula examples containing more than 30% of oil. Besides the fact that such amounts are not practical to maintain bond retention in a hot melt adhesive composition, it describes aromatic modified resins having a softening point approximately equal to or lower than 100° C., which leads to poor cohesion levels, non-aromatic modified resins having a softening point in a broad range of temperature (100 to 140° C.), which lead to bad adhesion levels, and use of pure aromatic resins with softening points greater than about 100° C., which is directionally opposite to the present invention.
EP 0 451 919 B1 and EP 0 451 920 propose polymer structures to coat adhesive at temperature as low as 121° C. These references exhibit formulas with specific polymers that are present at a level of 25% or more in the formulation, with an oil content from 0 to 25%, preferably from 0 to 15%. These conditions are remote from low viscosity products with the conditions the present invention proposes. No mention is made of the resin nature or softening point, except a general statement and examples that a 95° C. softening point resin is usable, and only mentioning that resins with softening points of 80 to 115° C. could be used.
WO 2004/035705 A2 covers the use of waxes, specifically microcrystalline waxes from 1 to 10%, to allow the viscosity of the composition to be lower than 10,000 mPa·s at 120° C., and to allow adhesive coating at below 120° C. No specific description of formulas is reported, but comparisons are made between compositions corresponding to different amounts and natures of wax. The reference discusses various test methods, i.e. a specific cube flow test, aged peel test, and G′ measurement which do not teach how to properly achieve bond retention in a given application.
WO 99/13016 presents a way to enhance specific adhesion, using a fatty acid oil and/or a natural oil in a hot melt adhesive composition. This allows for a lower application temperature as low as 100° C. to 130° C. This solution may not provide any economic advantage over current technology using conventional synthetic mineral oils, and does not teach how to make a hot melt composition that would be adapted to creep resistance or bond retention in general, and elastic attachment into a diaper structure in particular.
U.S. Pat. No. 5,275,589 proposes to apply hot melt adhesive at around 107° C. with the specific feature of obtaining a non tacky coating. The adhesive contains substantially no oil. Moreover, it is shown in the examples that low softening point resins are used to soften or make the polymer grades thinner. The targeted resin's softening point window, obtained by mixing several resins together, is from 25 to 50° C., which again is not compatible with an adhesive material getting acceptable creep resistance performances in general.
U.S. Pat. No. 6,465,557 B1 exhibits formulas where oil content is very high, higher than 25% and sometimes higher than 30%. Resins exhibited in the examples have a softening point of around 100° C., which prevents an acceptable bond retention of the adhesive bond at elevated temperature in the sense the present invention is showing it.
U.S. Pat. No. 6,184,285 B1 describes an adhesive composition having acceptable bond performances both at low and high testing temperatures. This uses a specific combination of polymer grades, and exhibits no high softening point resin, although it mentions that any conventional resin can be used. This reference does not focus specifically on solving creep resistance or bond retention issues.
US 2005/0176867 A1 claims formulas that are applied at 135° C., with no relevant mention or preference according to the softening point level of the cited tackifying resins, both midblock resins and end-block resins, which is a major characteristic of these ingredients to give cohesion to the final adhesive bond. Preferred midblock tackifying resins are said to have a softening point higher that 25° C., which is the majority of existing tackifying resins. Among a long description of conventional resins, mention is made of aliphatic petroleum hydrocarbon resins with softening points of from 70 to 135° C., which again is a very general description. Mention is also made of alicyclic petroleum hydrocarbon resins, aliphatic/aromatic or cycloaliphatic/aromatic resins and hydrogenated derivatives, with no mention of the softening points. Also, mention is made of preferred mid-block tackifying resins like Wingtack 95, Hercures C, Eastotac H100R, Escorez 5600, all having a softening point around 100° C. No mention is made about the aromatic/aliphatic nature of these resins, except that aliphatic ones are preferred. Concerning end-block tackifying resins, no mention is made about the softening point range that would be adequate to use.
WO 97/10310 focuses on the use of diblock structures to provide the right level of adhesion, without considering a specific and relevant domain for the tackifying resins' chemistry. Low to medium softening point mid-block resins are described, and no substantially aromatic resins are mentioned and none are exhibited.
WO 00/78886A1 claims adhesive formulas which all contain some additive like surfactants or polyether derivatives. Based on this peculiar feature, this reference does not bring any relevance to the present invention.
WO 98/02498 claims the use of wax materials to achieve low application temperatures in packaging applications. Moreover, resins mentioned in the examples have softening points of 100° C. or below, which does help with decreasing the viscosity of the adhesive, but not building enough cohesion in it. There is also some examples with a non aromatic 130° C.—softening point resin which do not show anything different from the examples where softening points are maintained at 100° C. or lower.
WO 2005/063914 A2 focuses on low viscosity hot melt adhesive using SIBS polymer and also including among other constituents mid-block tackifying resins, potentially having an aromatic character, and with softening point not higher than 95° C.
U.S. Pat. No. 6,818,093 B1 is very specific to construction applications in which some dermatologically-compatible coatings are present on substrates. This reference exhibit examples of formulas containing either low molecular weight polymer like a diblock structure SI, or low softening point mid-block resins. This interesting way to decrease cohesion and viscosity of the formulation does not correspond to what the present invention intends to do.
EP 0 798 358 B1 is focused on bottle labeling applications and claims low viscosity levels, and shows examples with high amounts of an oil plasticizer. Resins shown in the examples have softening points of approximately 100° C.
U.S. Pat. Nos. 5,266,394 and 5,143,968 mention the use of any tackifying resin having a softening point above 70 and below 150° C. They describe viscosity levels in the examples at 130° C., where the tackifying resin has a melting point of approximately 100° C.
EP 0900258 B1 is another reference describing some specific polymer features in a hot melt composition where only a 100° C. softening point resin is used. Interesting viscosity levels are reached at 130° C. but the compositions are outside of the domain the present invention.
Numerous references claim the concept of applying an adhesive at low temperature, with a certain lack of precision, i.e. they do not precisely define the temperature domain, or they do not give a clear way of how to practically achieve the low temperature application.
For example WO 98/02498 claims to achieve low application temperatures in the packaging area, which temperatures are intended to be lower than 150° C., but then also referred to be 135° C. and above. This is not sufficient to teach how to build cohesion in the adhesive material while having a low application temperature.
US 2005/0176867 A1 claims formulas that are applied at 135° C. with the use of conventional hot melt adhesive raw materials, and also with the use of additives as ionomer resins, for example for elastic attachment applications. 135° C. is not a low temperature for applying a hot melt adhesive, even for an adhesive based on thermoplastic elastomers like SIS, SBS and other polymers, as this temperature is currently used in many applications, including diaper manufacturing. In this reference, it is also mentioned that applications at a temperature as low as 93° C. can be achieved, but no description is given, even in the examples. In the examples, formulas are applied at 135° C., showing interesting creep resistance performances in elastic attachment, although no good indication is provided about adhesive add-on levels, as the width of the adhesive patterns is not specified. In the same examples, only where ionomer resins are used, it is stated that these formulas can be applied at temperatures lower than 135° C., but it is not shown, and it is further not shown that it could be done for elastic attachment. This reference is of no help in what the present invention intends to show, as it is mainly focused on the use of ionomer resin, and as it does not teach how to use hot melt raw materials in conjunction with elastic attachment made effectively at application temperatures lower than 135° C., and with effectively good creep resistance performances.
U.S. Pat. No. 6,465,557 B1 claims to run low temperature applications with no real precision on the temperature level achievable. The description of adhesive composition given in this reference does not allow one to think about a relevant way to get creep resistance from the bond with acceptable performances.
U.S. Pat. No. 6,184,285 B1 claims specific adhesive formulations that can be applied at about 135° C. or higher temperatures. Although it exhibits viscosity measurements at temperatures as low as 100° C., it does not show that this low level of temperature is achieved while applying the adhesive.
EP 0734426B1 focuses on low viscosity hot melt formulations but claims an application temperature of 150 to 200° C. This is outside of the domain the present invention.
WO 2005/063914 A2 claims low viscosity hot melt adhesives, with viscosities at or lower than 80,000 mPa·s at 177° C. Moreover, casting a film of adhesives materials in solvents is the method used for all the coating applications shown by this reference. No illustration of a potentially low process temperature for applying the adhesive is depicted, or discussed.
Aromatic resins, including pure monomer resins, are commonly used raw materials to formulate hot melt adhesives, those being PSA or not. The softening point of these materials is typically between 5° C. and 160° C., and their presence in the formulas can be driven by the level of tack and of adhesion required, as well as by the need to reinforce the styrenic phase of any styrenic block copolymer. Reinforcing resins help to provide a higher cohesion to the adhesive bond, at room temperature as well as at elevated temperature.
For example WO 97/19582 mentions the use of aromatic resins or pure monomer resins to reinforce the Tg of the styrene phase. This is very typical of the intent to use this kind of resin to enhance the level of cohesion of the adhesive material, with no emphasis on the level of viscosity that the presence of the resin is generating. Furthermore, there is typically no link with the need of applying the adhesives at low temperature. In this reference, no teaching is exhibited to help understand how or why low viscosity products made this way could be applied at low temperature.
WO 00/78886A1 mentions the use of Hercolite 290, which is a high softening point aromatic resin that helps to build cohesion into the adhesive material, as pure monomer resins are known for, but definitively does not help with lowering the application temperature.
The same remark can be said of US 2005/0181207, WO 2005/0182183 A1, and WO 2005/0182194 A1. Each of these references claim compositions for elastic attachment, which can use pure monomer resins of low/medium to high softening points, and specify a process temperature of 143° C. to 163° C. to apply hot melt adhesives according to the invention. This is evidence that no link has been made between low application temperature and low to medium softening point aromatic resins.
In US 2005/0176867 A1, aromatic resins, including pure monomer resins, are mentioned as a potential component of the adhesive formulas. First, these resins, as conventional ingredients widely used in typical hot melt formulation, are described in this reference in the same way other conventional ingredients are described like polymers, mid-block tackifying resins, and waxes, and no link is made to the fact that their presence is necessary or useful for their use in low application temperature formulations. Then, when it comes to a preferred hot melt composition in the detailed description of the invention, the use of a end-block compatible resin is mentioned, but only expressly in conjunction with the presence of an ionomer resin, from 0.1 to 40%. Finally, no mention or preference is made for the softening point value of the cited tackifying resins, both mid-block resins and end-block resins. Softening point of such tackifying resins is an essential characteristic of these ingredients in regard to low application temperature and is a key parameter for the present invention. These three last points show that the information disclosed in this reference is unable to teach one skilled in this art anything that would bring any relevance to the present invention.
None of the cited references claims any specific resin features based on composition, aromatic/aliphatic balance and softening point level to reach the right adhesion performances as described and claimed in the present invention. There is no relevance to be found in them according to the solution that the present invention has developed.